The present invention relates to a sheet feeding device which enables continuous sheet feeding to an image forming apparatus and the like which forms an image on a sheet, and an image forming apparatus provided with the aforesaid sheet feeding device.
When image formation is carried out using an image forming apparatus based on an image data, the image is transferred on the aforesaid sheet by feeding a sheet from a sheet feed tray in which sheets are stored. The aforesaid sheet feed tray is provided in an image forming apparatus. In addition, there also exist an apparatus in which the sheet feed tray is provided in a sheet feeding device separated from the image forming apparatus. Heretofore, when sheets are successively fed from the aforesaid sheet feed tray, in the case where it is required to feed more number of sheets than those in a sheet feed tray, the sheet feeding is continued until the sheet feed tray becomes empty. At this time, there is no intention to leave remaining sheets in a tray, and the issue is how to feed sheets without generating remaining sheets.
Further, there is also proposed a function to continue sheet feeding by switching to another tray having the same settings (a sheet size, a sheet type, and the like) as those of the current tray (an auto tray switch function) in the case where the number of sheets in the current sheet feed tray becomes insufficient. However, also in this case, the switching is carried out after the current sheet feed tray becomes empty.
In general, the sheet feed tray is provided with a sensor to detect existence or nonexistence of a sheet, and when the aforesaid sensor detects that there is no sheet in the tray, it is noticed to urge the operator to refill sheets.
In Unexamined Japanese Patent Application Publication No. H10-310285, a sheet feeding device is disclosed, which is capable of detecting sheets in three levels of number of sheet stored in a sheet feed tray being more than or equal to the prescribed one, less than the prescribed one, and the number of remaining sheet being zero, using a sensor to detect existence or nonexistence of a sheet and an adsorption sensor. That is, when the sheet sensor detects a sheet before and after adsorption of a sheet, the sensor detects that the number of remaining sheets is more than or equal to the prescribed one; when the sheet sensor detects no sheet after adsorption of a sheet, the sensor detects that the number of remaining sheets is less than the prescribed one; and when the sheet sensor detects no sheet before adsorption of a sheet, the sensor detects that the number of remaining sheets is zero. However, even in the above device, in the case where, at a continuous sheet feeding, the number of sheets required exceeds the number of sheets capable of feeding, the sheet feeding is continued until the sheet feed tray becomes empty.
When sheets are refilled after the sheet feed tray becomes empty such as described above, it is generally required to refill sheets of the same kind (such as a sheet type, and a sheet size). But, recently, since, in markets such as a POD (a print on demand) market, a plenty of sheet types exists and various sheet types are used, and further, there exist a trend of a larger apparatus such as a multi-sheet feed unit and increase in the number of sheet feed trays, it becomes difficult to select a required sheet from a plurality of sheet types. Consequently, there exists a case where it is difficult to determine a sheet type (for example, a sheet type, a color, a setting orientation (a back and front, top and end, and left and right)), based only on sheet information displayed on an operation screen of a device, resulting in a different sheet being likely to be set before and after the sheet refill. In particular, for papers for additional printing, coated papers, back side reuse papers, papers for label, processed papers, and the like, when a wrong sheet is set, an image is not formed at an intended part, and as a result, the above paper sheets become wasted ones.
Further, when a sheet feed tray is away from a sheet storage place, it is also difficult in terms of security or quality preservation of a printed sheet to take away printed sheets and bring it to the sheet storage place to compare it with sheets for refilling.
For coping with the problems, an image forming apparatus has been proposed (Unexamined Japanese Patent Application Publication No. 200-234490) in which, when an amount of remaining sheets in a sheet feed tray reaches an appropriate one, printing operation is stopped. In the above image forming apparatus, when a back surface of a sheet, in which printing is completed on one surface, is used for a printing surface, it makes possible for the user to refill right sheets by observing the orientation (upward or downward) of a sheet remained in a sheet feed tray because the above printing operation is stopped. Therefore, based on the above constitution, it becomes possible to confirm, by using a remaining sheet, a sheet type or a sheet size.
However, since in the image forming apparatus, which was disclosed in the above Unexamined Japanese Patent Application Publication No. 2005-234490, the printing operation stops when the number of remaining sheets reaches an appropriate one, the image forming apparatus remains stopped unless a user confirms a sheet in the sheet feed tray. For this reason, unless, for example, a user knows that the apparatus remains stopped, the apparatus remains stopped for a long time, resulting in a problem of significant decrease of production efficiency.